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zlib: reach the z_stream only through a raw pointer while zlib-ng holds its back-pointer - #46607
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…ds its back-pointer zlib-ng stores the z_stream pointer in its internal state at init and reads and writes the stream through it inside every later call. Rust formed a `&mut zStream_struct` (or a `&mut self` over a struct that held one inline) and kept it alive across those calls. That foreign access to a protected borrow is undefined behaviour under Stacked Borrows and Tree Borrows. Add `ZStream`, a heap-pinned z_stream behind a NonNull. It never hands out a reference to the stream: field reads and writes go through the raw pointer, and the C calls take `as_ptr()`. `ZlibReaderArrayList`, `ZlibCompressorArrayList`, `DeflateEncoder`, `InflateDecoder`, and the node:zlib `Context` all hold a `ZStream` now. The ArrayList wrappers no longer need to be boxed, since the stream is pinned on its own.
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info
WalkthroughThe change adds a ChangesZlib stream ownership migration
Possibly related issues
Suggested reviewers: Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to
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| Check name | Status | Explanation |
|---|---|---|
| Title check |  |
The title clearly and concisely describes the main change: accessing z_stream through a raw pointer while zlib-ng retains its back-pointer. |
| Description check | |
The description explains the problem, implementation, behavior impact, verification steps, known test limitations, and tradeoffs. It does not use the template headings exactly, but it provides the req… |
| Linked Issues check | |
Issue #46598 requires that zlib not read the z_stream while Rust holds a live &mut zStream_struct across a zlib call. The reviewed head adds ZStream in src/zlib/stream.rs. It stores the `z_strea… |
| Out of Scope Changes check | |
The changes follow from the #46598 fix or support it. Moving every stream owner to ZStream applies the same aliasing rule to each producer. Removing the Box<Self> return from the ArrayList wrapper… |
Full details: Description check
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Explanation
The description explains the problem, implementation, behavior impact, verification steps, known test limitations, and tradeoffs. It does not use the template headings exactly, but it provides the required information in equivalent sections.
Full details: Linked Issues check
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Explanation
Issue
#46598requires that zlib not read the z_stream while Rust holds a live&mut zStream_structacross a zlib call. The reviewed head addsZStreaminsrc/zlib/stream.rs. It stores thez_streamin a heap allocation behindNonNulland never returns a reference to it. Field reads and writes (next_in,avail_in,avail_out,total_out,err_msg,set_input,set_output) go throughas_ptr().deflate_initandinflate_initpassas_ptr()to zlib with the linked version and struct size. The PR description states thatDeflateEncoder,InflateDecoder, both ArrayList wrappers, the node:zlibContext, and the WebCompressionStreamcoder now own aZStream, and thatbun_zlibno longer re-exports the rawz_stream. The&mutborrow over each owner now covers only the pointer field. The author reports a Miri model that passes under Stacked Borrows and Tree Borrows, and reports passing zlib, compression, fetch-gzip, and websocket-permessage-deflate test suites on a debug build. This assessment did not rerun Miri, BorrowSanitizer, or the test suites. The owner-level call sites insrc/zlib/lib.rsandNativeZlib.rswere assessed from the change summary and the description, not from a line-by-line read. The coding requirement is met by the reviewed code shape.
Full details: Out of Scope Changes check
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Explanation
The changes follow from the
#46598fix or support it. Moving every stream owner toZStreamapplies the same aliasing rule to each producer. Removing theBox<Self>return from the ArrayList wrappers follows from pinning the stream separately. Removing the untaggedmi_malloc_itemsandmi_free_opaquethunks insrc/bun_alloc/c_thunks.rsfollows from movingZlibCompressorArrayListonto the zone-tagged zlib thunks. The diff forc_thunks.rsconfirms those removals. The macro change inc_thunks_for_zone!(onezone()accessor shared by the alloc and free thunks instead of twoget_zone!expansions) is a separate bug fix. It is disclosed in the PR description. It also touches the zone thunks thatZStream::newnow installs on every zlib stream (ZlibAllocatorusesc_thunks_for_zone!("zlib")), so the alloc and free paths must agree for the new owners. The description also discloses thetotal_in/total_outinitialization change, theZlibCompressorArrayList::initnext_outchange, and the WebCompressionStreamallocator change. These are tied to the owner migration. No unrelated change is demonstrated.
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Updated 11:50 AM PT - Oct 23rd, 2026
✅ @robobun, your commit 4c243d981509d168c95493feb49e013bca884528 passed in 🧪 To try this PR locally: bunx bun-pr 46607That installs a local version of the PR into your bun-46607 --bun |
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Beyond the inline findings, I also checked the behavior deltas in the ArrayList constructors: dropping the pre-init next_out in ZlibCompressorArrayList::init is safe because read_all installs the output window from spare capacity before every deflate (and the old pointer would have been stale after try_reserve_exact anyway), the removed total_in/total_out seeds are overwritten by deflateInit2_/inflateInit2_, and returning Self instead of Box<Self> is fine since zlib-ng's back-pointer targets the separately heap-allocated z_stream, not the owner.
Extended reasoning...
The change moves every zlib stream owner in src/zlib/lib.rs and src/runtime/node/zlib/NativeZlib.rs onto a new heap-pinned ZStream wrapper in src/zlib/stream.rs so no &mut to the z_stream is live across FFI calls; it touches no auth, crypto, or input-parsing surface beyond the existing decompression paths. The refactor side effects on the ArrayList constructors (pointer seeding, total counters, Box removal) were traced and are behavior-preserving.
Findings marked 🟡 are optional suggestions and need no follow-up push.
… allocator thunks CompressionStreamCoder still held Box<z_stream>. It now holds ZStream like every other owner. The untagged zlib allocator thunks in bun_alloc lost their last user, so remove them.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
The follow-up commit addresses the CompressionStream owner, the untagged thunks, and the Drop SAFETY comment from the earlier round. Beyond the inline note, I also checked that the three callers of the ArrayList init functions (BunObject.rs, StandaloneModuleGraph.rs, create_command.rs) bind the result without a Box annotation, that ZlibCompressorArrayList::read_all sets the output window before every deflate so dropping the init-time next_out changes nothing, and that NativeZlib::drop calls close() before Context's ZStream frees the z_stream.
Extended reasoning...
The second push migrated src/runtime/webcore/CompressionStreamCoder.rs onto ZStream, deleted mi_malloc_items/mi_free_opaque from src/bun_alloc/c_thunks.rs, and corrected the SAFETY comment in src/zlib/stream.rs. This run's remaining concerns were the changed init return types, the removed init-time next_out, and drop ordering for node:zlib handles; each checked out from the code. The change is an FFI aliasing refactor across five files with no automated test, so a human look is still appropriate.
Still open from earlier reviews (2):
- Unresolved: 2 minor or pre-existing.
ZStream::deflate_init and ZStream::inflate_init carry the zlib version and struct size, so no owner names zStream_struct any more. bun_zlib no longer re-exports z_stream. Shorter comments in stream.rs.
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The CompressionStreamCoder owner and the Drop SAFETY wording from the earlier pass are now in line with the rest of the migration. Beyond the inline finding, I also checked the init-failure paths that lost their explicit drop(zlib_reader) in src/zlib/lib.rs (try_reserve_exact failure and the non-Ok init codes): both ArrayList wrappers still run their owner Drop, which calls deflateEnd/inflateEnd before ZStream frees the cell, so no zlib-ng state leaks there. The four external callers of ZlibReaderArrayList::init/ZlibCompressorArrayList::init only use method calls, so the Box<Self> to Self return change is source-compatible.
Extended reasoning...
The diff adds a heap-pinned ZStream wrapper and rewires five zlib owners (ArrayList wrappers, DeflateEncoder/InflateDecoder, node:zlib Context, Web CompressionStream coder) to reach the z_stream only through a raw pointer, deleting the untagged allocator thunks and the z_stream re-export. It touches unsafe FFI and allocator pairing but no auth, injection or data-exposure surface. A human look is still worthwhile because the change is a cross-cutting unsafe refactor with no committed test and one confirmed (pre-existing) allocator-zone finding posted inline.
c_thunks_for_zone! expanded get_zone! once in malloc_size and once in free. Each expansion is its own static, so on a macOS debug build without ASAN the free thunk used a different zone than the alloc thunk.
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Actionable comments posted: 1
- 🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @src/zlib/stream.rs:
- Around line 67-68: Add an automated aliasing regression test for
DeflateEncoder that initializes the retained zlib back-pointer and exercises it
through DeflateEncoder::step. Ensure the test detects the pre-fix borrowing
behavior without changing unrelated code.
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- Branch: robobun/3b69a136/zlib-stream-raw-pointer
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📒 Files selected for processing: 5
src/bun_alloc/c_thunks.rssrc/runtime/node/zlib/NativeZlib.rssrc/runtime/webcore/CompressionStreamCoder.rssrc/zlib/lib.rssrc/zlib/stream.rs
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I re-reviewed the latest push (4c243d9) and found no new bugs; the points raised in earlier rounds are now addressed in the code, but because this introduces a cross-cutting unsafe/FFI ownership type used by node:zlib, CompressionStream, and Bun.gzipSync/gunzipSync, a human look is still worthwhile.
What was reviewed:
c_thunks_for_zone!now resolves onezone()per label, so alloc and free share a single malloc zone;get_zone!is otherwise unused.- Every
z_streamowner (both ArrayList wrappers,DeflateEncoder/InflateDecoder, node:zlibContext,CompressionStreamCoder) reaches the stream only viaZStream::as_ptr()/accessors; no leftover field access,&raw mut, orz_streamre-export consumer remains in src/. - Drop ordering checked:
deflateEnd/inflateEndruns in each owner's Drop (orNativeZlib::drop->close()) before theZStreamfield frees the struct; init-failure paths hit*Endon a nullinternal_state, which is a defined no-op. - Callers of the
Result<Box<Self>>->Result<Self>change (BunObject.rs, create_command.rs, StandaloneModuleGraph.rs) use the value by method call only;mi_malloc_items/mi_free_opaquehave no remaining users. The diff still ships no automated test.
Extended reasoning...
The change adds src/zlib/stream.rs (a heap-pinned z_stream behind NonNull with accessor methods) and migrates all five zlib stream owners to it, deletes two allocator thunks, and fixes the zone thunk macro to create one malloc zone per label. It touches no auth, crypto, or injection surface, but it is unsafe/FFI memory-ownership code on paths reachable from user input (node:zlib, CompressionStream, Bun.gzipSync) and silently changes the allocator CompressionStream and node:zlib hand to zlib-ng. The bug hunt ran dry and my own read confirmed drop ordering, caller migration, and dead-code removal, which argues against silence now that the last commit addressed the previously raised zone-split finding. Deciding against approve: the PR introduces a new cross-cutting abstraction, the diff carries no automated test, and several review threads were resolved only by the author.
Fixes #46598
Behaviour change: none
Problem
z_streamin its internal state at init (vendor/zlib/deflate.c:299s->strm = strm,inflate.c:247) and reads and writes the stream through it inside every later call (fill_window:s->strm->avail_in,deflate_stored.c:s->strm->next_out).&mut zStream_structacross those calls:fn step(strm: &mut zStream_struct, ..)insrc/zlib/lib.rs, and&mut selfmethods over structs that held thez_streaminline (ZlibReaderArrayList,ZlibCompressorArrayList, node:zlibContext). A foreign access to a protected&mutis undefined behaviour under Stacked Borrows and Tree Borrows. BorrowSanitizer reports it onDeflateEncoder::step. Same bug as Tree borrows violation occurs when using zlib backend rust-lang/flate2-rs#392.Fix
ZStream(src/zlib/stream.rs): a heap-pinnedz_streambehind aNonNull. It never hands out a reference to the stream. Field reads and writes go through the raw pointer,deflate_init/inflate_initcarry the zlib version and struct size, and every other call into zlib takesas_ptr().bun_zlibno longer re-exports the rawz_stream.ZStream:DeflateEncoder,InflateDecoder, both ArrayList wrappers, node:zlibContext, and the WebCompressionStreamcoder. A&mutover the owner now covers the pointer field only, so the&mut selfmethods stay.Box<Self>. They were boxed so the inline stream did not move. The stream is pinned on its own now.test/js/node/zlib/*,test/js/web/streams/compression.test.ts,test/js/web/fetch/fetch-gzip.test.ts,fetch-compress.test.ts, and the threewebsocket-permessage-deflate*files pass on the debug build.c_thunks_for_zone!(src/bun_alloc/c_thunks.rs) created two malloc zones per label, one in the alloc thunk and one in the free thunk. It now creates one. Only a macOS debug build without ASAN reaches that code.CompressionStreamCoderowner, the dead allocator thunks, a wrong helper name in a SAFETY comment, init helpers onZStream, thez_streamre-export, the split malloc zone).Background
z_streamis zlib's public stream struct. The caller setsnext_in/avail_in/next_out/avail_outand callsdeflate/inflate. zlib keeps a pointer to that struct in its private state and uses it in every call, so the struct must not move and Rust must not hold an exclusive borrow of it while zlib runs.scripts/rust-miri.ts). The old shape fails under it as well as under Stacked Borrows.&mut Tparameter isnoaliasto LLVM. Release builds link Rust andvendor/zlibwith cross-language ThinLTO, so oncedeflate()inlines intostep()LLVM can reorder the C side's stores throughs->strmagainst the Rust side's loads.steponly). It leaves the three inline-z_streamowners with the same shape and nothing stops a later method from forming a&mutagain. One owning type puts the rule in the place every producer passes through.Downsides
z_streamwas inline in the handle before). The other owners allocate the same count as before: one per stream.CompressionStream/DecompressionStreamnow give zlib-ng Bun's allocator thunks instead of a nullzalloc(zlib-ng's ownmalloc). The internal state lands on the same heap as every other zlib stream in Bun.Box<zStream_struct>had. Binary size not measured: no release build of the merge base in this container, andbloatyis not installed.Notes
zlibcrate #46598 runscargo bsan test -p bun_zlibagainst asrc/zlib/tests/repro.rsthat callsDeflateEncoder::new(6, 15 + 16, 8, 0)thenstep(.., FlushValue::Finish).Box<Stream>owner,step(&mut self.strm), C reads(*(*s).strm).avail_in) fails under Stacked Borrows ("not granting access to tag because that would remove [Unique] which is strongly protected") and under-Zmiri-tree-borrows("the accessed tag is foreign to the protected tag"). The new shape (NonNullowner,&mut Contextwith the owner inline,step(&mut ZStream),&selfgetters) passes both.ZlibCompressorArrayListused the untaggedmi_malloc_itemsthunks. It now uses the same "zlib" heap-breakdown zone thunks as every other stream. The bytes go to the same allocator. Those untagged thunks had no other user, sobun_alloc::c_thunksdrops them.ZlibCompressorArrayList::initno longer pointsnext_outat the list beforedeflateInit2_.read_allsets the output window from the list's spare capacity before eachdeflatecall, andtry_reserve_exactininitcould move the list after that first pointer was taken.total_in/total_outwere set to the input and list lengths in the ArrayList inits.deflateInit2_/inflateInit2_reset both to 0 (deflate.c:493,inflate.c:67), so the values were never observed.test/js/web/fetch/fetch.stream.test.tsfails in this container on the tests after thetimeout 100case. The same tests fail the same way with the released bun, so it is the container, not this diff.no test proof · iteration 0 · the description declares no behaviour change, so there is no failing test to prove; the existing suite in CI is the check